Fuel element for a neutronic reactor



Aug. 13, 1963 R. K. MCGEARY ETAL 3,100,742

FUEL ELEMENT FOR A NEUTRONIC RECTOR Filed March 17, 1958 Fitz R.Winslow.

If/ BY wj United States Patent C) The present invention relates to afuel element assembly for neutronic reactors and more particularly to afuel assembly adapted for use in a pressurized water heterogeneous typereactor. A

The tuel element disclosed herein is adapted for use Vin i a neutronicreactor wherein a quantity or" one or more of the issionable isotopes11233, U235 and Pu239 is caused to undergo a chain reaction. The chainreaction is propagated by neutrons having a specific range of velocitiesdepending upon the type of reactor in which the iissionab-le material isemployed.

ln a thermal-type reactor the chain reaction is propagated by neutronshav-ing velocities equivalent to thermally excited hydrogen ions orabout 2500 meters per second. A neutronic reactor of this characterusually employs the U233 or U235 isotopes or both of uranium topropagate the reaction. However, during the tissioning of each of theseatoms, which, of course, results in two approximately equal fissionfragments, an average of approximately three neutrons are expelled.However, most oi these neutrons have velocities in the fast region, thatis to say in excess of 10,000 meters per second. Therefore, in

l order to propagate the` chain reaction in -a thermal reactor, it isnecessary to employ a neutron-slowing `material, termed a moderator, toslow the fast neutrons to the thermal velocity region. Suitablemoderator materials are carbon, hydrogen, and deuterium, or materialscontaining thcsc elements such as graphite, light water, deuterium oxideor heavy water, and radiation-stable organic materials, such as themulti-phenyls.

The thermal-type reactor may be constructed in the form of a latticenetwork of uranium or an alloy thereof containing one of the iissionableisotopes and provided with coolant passages for the `circulation `of asuitable cooling medium such as water or carbon dioxide. Alternatively,the nuclear fuel material can be suspended in spaced relation in apressurized vessel, and light or heavy water can be utilized both as acooling medium and as a moderator material, with suitable circulatingloops being employed to maintain a iiow of the moderator-coolant throughthe vessel to remove the heat developed by the chain reaction. Suitableheat exchanging means are disposed in the aforementioned circulatingloops in, order to convert into steam the water supplied to the heatexchanger from one or more secondary or thermodynamic loops. Y

As Vstated previously,` the aforementioned coolantmoderator, in certainapplications,` can be either light or heavyV water, with light waterbeing the more efficient moderator material due to the lighter weight oithe hydrogen nuclei. On the other hand, the use of heavy water isapplicable in those cases wherein it is necessary to minimize theparasitic absorption of ssional neutrons within the reactor system as aresult of peripheral and structural neutron losses. Heavy water isadvantageous in this application in that it has a much smaller neutroniccapture cross-section than that of light water.

The peripheral loss of neutrons from the chain reacting system isminimized by providing a neutronic reflector adjacent the periphery ofthe system or adjacent the inner walls of the containment vessel. Thisreflector can take the form of a layer of graphite disposed adjacent theinner wall surface ofthe vessel or of a water chamber or an- 3,100,742Patented Aug. 13, i963 ICC nular space usually reserved for this purposebetween the outer periphery of the reactor core structure and the innerwall of the reactor vessel. When light or heavy water is employed as acoolant, this annularispace, of course, is liiled with the water andthus serves as a neutronic shield or reflector.

Another form of neutronic reactor is typified by the so-calledepithermal reactor. In this reactor a. `quantity of at least one of theaforementioned iissionable isotopes are subjected to a chain reactionmaintained by neutrons within the epithermal region, that is to say,neutrons having velocities between 2500 and 10,000 meters per second. Inthis form of reactor, the nuclear fuel or iissionable material isrelatively closely spaced in order to lessen the moderating capacity ofthe moderator material. This form of reactor is particularly applicablefor the iissioning of plutonium 239 which is iissionable most eiicientlyby neutrons in the higher velocity ranges. i

The thermal or epithermal-type reactors are controlled by means ofso-called control rods which are insertable through the reactor vesselinto the reactor core. These control rods each contain a relatively goodneutronabsorbing material, such as boron, hafnium, and gadolinium, andby variably absorbing the neutrons produced by the chain reaction,depending upon the position of the control `rods relative to the reactorcore, the propagation of thechain reaction to the reactor core, can becarefully controlled. j

yIn any of the aforementioned reactor systems, a fertile or blanketmaterial, such as uranium 238 or thorium 232, can be mixed orintermingled with `the previously mentioned tissionable materials orotherwise disposed withinthe reactor vessel at positions adjacent to theiissionable isotope.` The fertile materials, when thus used, absorbthose neutrons which are `not required for the propagation ofthe chainreaction and in doing so are transmuted into certain ones of thetissionable iso-i topes. t .For example, during neutronic radiation,atoms of the fertile material uranium 238 when employed arejconvertedinto the ssionable transuranic element plutonium 239 in accordance withthe following nuclear equations:

'with the timesdenoted in connection Withthe latter two reactions beingthe half-lives of the decaying isotopes.`

The plutonium 239 isotope is producedtin either the `thermal reactorwherein the initial core loading consists of natural or source-gradeuranium, which may be enriched The artiiicial fission-able isotope U233and the transuranic (Keff) is equal to runity and as a result, of theaverage about 12 minutes after vthe fission process.

en average of about 2.8 to 2.9 of the 3.0 :fission neutrons are emittedimmediately upon fissioning of each chainreacting atom. The delayedneutrons are emitted up to Although only about 3 to 7% of the totalneutrons emitted as a result of atomic fission are delayed neutrons, thetimemargin provided by these delayed neutrons is suicient to permitadjustment of the control rods to maintain the chainV reactionapproximately at the point of criticality during operation of thereactor. Otherwise, aV tendency to an increase lin the number oflissions from one generation to the next in the chain reaction would bemultiplied almost instantaneously.

A more detailed description of the theory and operation of neutronicreactors are given in Patent Nos. 2,708,- 656 and 2,798,847, issued May17, 1955, and July 9, 1957,- to Enrico Fermi and Leo Szilard, andentitled Neutronic Reactor and Method of Operating a Neutronic Reactor,respectively.

In orderfto protect the lissionable material fromY the corrosive effectsof the high temperature pressurized water utilized in the coolant systemfor neutronic reactors of the character described herein, it isnecessary to encase the the pellets, that is to say in a directiontransversely of the tube walls, will not cause binding between one ormore of the pellets and the cladding tube.

In known arrangements of reactor fuel rods, however, the aforementioneddifferential thermal expansion necessitates the provision of .a voidbetween the upper end of the column and the upper end of the claddingtube to accommodate the aforesaid relative longitudinal movement of thepellet column. The length of this void is dictated by the length of thepellet column and must be approximately 0.01 inch per `inch of .pelletcolumn length in order to absorb the aforesaid differential expansion.For

those elongated fuel rods mentioned previously, the length' of the voidtherefor is approximately 1 inch.` In the poses, this pressure may reachla value of 2500 pounds l per square inch during operation of thereactor and 3750 p.s.i. during preoperational testing. void exceeds Vthediameter of the cladding tube, collapsing will occur in the :area of thevoid unless the cladding tube is made sufciently strong to withstandthis pressure. As a result, the drawing techniques employed infabricating v the cladding tubes necessitates making the entire tubewall fssionable isotope within a suitable corrosion-resistant Y claddingmaterial. Because of the considerable length of the fuel rods employedwithin the reactor core, the cladding material land associatedstructural components have heretofore been miade in relatively largesizes or thicknesses in order to support properly the' fuel elements.-In many cases, the required mass of structural material increasedtheimportance of utilizing material having a low neutronic absorptioncross-section.

"The fuel rods employed in the present or proposed pressurized waterreactors and similar heterogeneous type reactors frequently areprovidedwith -a total length in theneighborhood of eight to ten feet and with anoutside diameter of about 0.3 inch. As described more fully hereinafter,vthese fuel rods comprise an elongated tube of acorrosion-resistant'material in the casefof water cooled reactors, forexample, stainless steel, zirconium, or

av zirconium alloyoraluminum alloy. Althoughstainless steel ispreferable insofar as handling and fabricational characteristics areconcerned, this material as a relatively higher neutronic absorptioncross-section. Accordingly, it is imperative that the tubing orVcladding be made as thin as feasible in order to minimize parasiticabsorption of neutrons therein. v The aforementioned cladding tubes arethen substantially filled with `a quantity of one of the aforementionedYSuchV clearance is necessary to permit insertion of the Ypellets withinthe tube in order to form a column of ellets extending therethrough.Moreover, during operation of. the reactor, the pellets being at ahigher temperature that the cladding tube, which is in contact with thecoolant flowing throu-gh the reactor, expand to a greater extent-wandtherefore the aforesaid Aclearances permit longitudinal movement of thepellet column relative to` the cladding tube.

mustbe large enough that relative radial expansion of However, theseclearances thicker than that required merely for supporting of the fuelrod. However, if the continuous unsupportedVv length of the .aforesaidvoid can be reduced, resistance to collapsing increases and it ispossible to utilize fuel rods having thinner cladding. The usage of athinner cladding material is made feasible by the present invention asdescribed hereinafter. affords an improvement in neutronic economy ofthe reactor, thereby 'permitting the use of less expensive claddingmaterial such as stainless steel.

As stated previously in known types of reactor fuel rods, it isnecessary to preserve a certain amount o-f Y clearance between the fuelpellets andthe cladding tube in order to permit relative movementtherebetween. How,-

ever, usage of such clearances increase'the heat transfer coefficientbetween the fuel pellets and the reactor coolant. As ya result, themaximum temperature at whichV the reactor core can be operated isreduced, since the temperature adjacent the axis ofthe pellet lcolumnisincreased.

It should be added, however, that .the pellet diameter also increases atthis time relative tothe inside diameterof the cladding tube an-dthereby reduces to some extent these clearances. However, the clearanceis not eliminated all together, and there is no assurance that therelative increases inthe pellet diameter will be uniform. Moreinner wallof the cladding tube during'rcyclic operation of the reactor.

Moreover, the resultant relative movement ofthe fuel pellets relativevto the cladding tube of known fuel rod arrangements produces eventuallya number of chips and Vother particles of the fissile oxide, which,during a nurnber of operating cycles of the reactor, may accumulate 1nthe clearances described previously. As la result, the frictionassociated with relative longitudinal movement between the pellet columnand the cladding tube of prior tween portions of the fuel pellet columnand Vthe adjacent inner wall of the cladding tube. Continued expansionof If the length of theV As a result, the present invention I t thecolumn then results in a slight stretching of the cladding tube beyondits elastic limit er yield point. During repeated operation of cycles ofheating and cooling of the reactor fuel rods, the cumulative result ofsuch inelastic stretching can eventiiallycause some of the fuel rods tobuckle or to rupture with attendant release of highly radioactivelissional products into the primary coolant system. This disadvantage ofkno-wn types of fuel rods is likewise obviated by `the invention whichaffords means for causing the cladding tube to grip securely each one ofthe fuel pellets suclrthat each pellet can expand and contractindependently of the remaining fuel pellets.

In view of the foregoing, an object of the present invention is toprovide a novel and eiiicient fuel element adapted for use in aneutro-nic reactor.

Another object of the invention is to furnish a novel and improvedmethod for fabricating a reactor fuel element.

Further objects of the invention are to facilitate passage i of coolantllowing between adjacent fuel elements and to minimize the spacingrequired for adequate coolant flow therebetween.

Still another object of the invention is to provide a fuel element ofthe character described wherein means are provided for increasing therate of heat transfer from the fuel material thereof to the outersurface .of the Vfuel element.

Still another object of the invention is to provide a -fuel elementwherein the clearances between the fuel material and the claddingmaterial are eliminated.V

A further object of the invention is to provide a` reactor fuel elementwherein the `fuel material thereof is .provided in the form ofindividual pellets which are gripped independently by the claddingmaterial of the fuel rod.

A still further object of the invention is the provision of a reactorfuel element having means for reducing the required thickness ofthecladding material therefor. These and other objects, features andadvantages of the vinvention will be described more fully `hereinafterduring the forthcoming description of lan illustrative modification ofthe invention, with the description being taken in oorljunction with theaccompanying drawings, wherein:`

FIGURE 1 is a longitudinally sectioned view of a fuel rod preassembly towhich the method of the invention is adaptable; and

FIG@ is a longitudinally sectionedview of Va rod type fuel rodarrangement in accordance with the invention. The fuel element disclosedand claimed herein is adapted for incorporation in a fuel elementassembly such as that shown and described in copending applications ofErling Frisch, entitled Fuel Element Assembly for a Nuclear Reactor,Serial No. 635,911, now abandoned, and Neutronic Reactor Fuel Element,`Serial No. 721,- 775, tiled January 23, 1957, and March 17, 1958,respectively, with both of these applications *being assigned to thepresent assignee. Thus, a plurality` of the fuel rods disclosed hereinare adapted for joining laterally to one another and at their ends to apair of flow nozzles `or end pieces, respectively, as described in theaforementioned cjopending applications.

The fuel element disclosed and claimed herein is adapted also :for usefor a neutronic reactor such as thatdescribed in detail in a paperentitled Description of the Pressurized Water Reactor (PWR) CompletedPower Plant at Ship-pingport, Pa, by I. W. Simpson et al., A/Conf.SHP/815, published by the United Nationsas part tof the Geneva Papers.More particularly, the aforesaid fuel element assembly is adapted foruse in the neutronic reactor described ina paper entitled The YankeeAtomic Electric Plant, by Glen A. Reed, Robert J. Creagan, and Walter C.Woodman, presented at the A.S.M.E. annual meeting in New York City,November 26 to 30, 1956. The lasomentioned neutronic reactor is alsodescribed in a copending application of Robert J. Creagan, entitledNeutronic Reactor, Serial No, 686,-

778, filed September 27, 1957, now abandoned, and assigned to theassignee of the presentapplication. The over-all configuration of thefuel element assembly forming the subject of the present invention issubstantially similar to that described in the aforementioned ASME.publication, with the exception that the fuel element assembly isprovided with an improved and novel lateral and end joining means.

Referring now more particularly to the drawings, the illustrative formof the invention disclosed therein comprises yan elongated cladding tube2d fabricated, in the oase of a pressurized water type heterogeneousreactor from a corrosion resistant material such as stainless steel.Zirconium or an alloy thereof or other structural material haping arelatively low neutronic capture cnoss-section might also be utilized;however, the use of stainless steel is desirable in view of itsrelatively lower first cost and superior fabricational characteristics.A plurality of fuel pellets 22 are inserted within the cladding tube:20. The fuel pellets 22 desirably are right-cylindrical ceramic compactswhich, for example, comprise uranium dioxide (U02). Initially, theceramic pellets 22 form a continuous column as better shown in FIGURE -1rofl the drawings extending substantially throughoutthe interior the:cladding tube 20.

At the bottom end 28 of the cladding tube 20 the column of pellets 22rests upon the inward end of a plug member 30, la neck-down portion32":of which is inserted into the end 28 of the tcl-adding tube 2li. Asimilar plug member 3dis inserted into` the upper end Z6 of the claddingtube 20, and each ofthe end plugs 30 and 34 are hermetically sealed tothe adjacent end ofthe cladding tube 2t) by means of annular sealingwelds -36 and 38, respectively. In the fuel` rod arrangement inaccordance with the invention, however, provision of the aforementionedvoid employed in known fuel elements between the uppermost fuel pellet22a and the inward end of the plug member 34 is not necessary.;` Thus,the upper surface ofthe fuelpelletla can rest against or be .dis-

posed very `close to the inwardly extending neclodown portion id of theupper end plug 34. in this manner, the fuel pellets `22 are hermeticallysealed within the cladding tube 29 in order to prevent the escape oftissional products therefrom intothe reactor coolant stream. The outwardends of the plug members 30 and 34 desinably are tapered or chamfered asdenoted by reference characters 4d to facilitate the flow of reactorcoolant moving longitudinally of the `fuel nods. f 1

To aid in inserting the fuel pellets 22 into the cladding tube 2li,before the upper end thereof is closed by the plug member 3d, each ofthe fuel pellets 22 `are furnished with a ydiameter less than the innerdiameter of the cladding tube 2t) so that a clearance denoted byreference character 42 initially is provided between the fuel pellets Z2and the adjacent inner wall of the cladding tube 20. In thisarrangement, the fuel column .is of the order of 100 inches in lengthin-iti-al-lyfand extend-s substantially the entire length of the`cladding tube 2), inasmuch as assemblies of `approximately 305 fuelrods and each are assembled in a square lattice array between stainlesssteel core platesas ydescribed in 'the aforementioned refermoderatorcoolant material.

pellets 22 .totaling 102 inches. The reactor core loading then comprises24,400 kilognams based on free column and having yan approximateenrichment of 2.6% of the U23? iso-tope. The reactor core then isdesigned for calculated reactivity life of 10,000 hours.

As described in the aforementioned A.S.M.E. document and copendingapplications, the fuel elements are supported within the assemblies (notshown) vthereof on 0.425 inch centers, while the individual assembliesor bundles of fuel Ielements are spaced on 7.61 inch centers,

VThe containing vessel (not shown) is generally cylin# drical with aninside height and inside diameter of thirty feet and nine feet,respectively.

The reactor coolant is circulated through the vessel in la known manner`and at the rate of 37,000,000 pounds per hour. With this arrangement amoderator-rto-fuel ratio at standard conditions (STP), ass-uming amoderator-coolant of light water (H2O), of 3.03 is attained.

, A nentronic reflector is afforded by `an annular space of y13 inchesin radial thickness provided between the outer periphery of the reactorcore and the inner wall surface of the reactor vessel and filled withthe reactor This space communicates with Ithe main coolant stream` ofthe reactor vessel,` as shown and described in the aforementionedCreagan application, fto prevent'boiling in the reflector area.VV'Iliisf of fuel pellets shown fin FIGURE `1 in rthis example, then, isabo-ut 102 inches.

However, .the total iactive length of the fuel pellets in Ithe :finishedfuel rod of the invention is approximately fone inch llonger asdescribe-d hereinafter. In an exemplary arrangement of :the inventionthe cl-adding tube 20 is supplied initially with an linside diameter of0.304-i.001 inch iand a length of about 106 inches. The fuel pellets 22on the other hand `are `machined such that the diameter of theircylindrical .surfaces equals .3000120005 inch. The diameter of thenecked-down portions 32 and 42 of the end plugs 30 land 34,respectively, are machined to fit `relatively closely in .the endportions of the cladding tube L The height or length of each fuelVpellet 22 is of the order of 0.6` inch. With this arrangement, then, aninitial diametric clearance `42. of .003 to .006 inch is affordedbetween the fuel pellets and the cladding (tulbe 2.0. The initialassembly then of the fuel rod lof the invention is similar to that ofknown arrangements with the exception that a void or space is notprovided between the `uppermost fuel pellet 22a and the inward end ofthe Iupper end plug 34.

l VVReferring now to FIGURE 2 of the drawings, the final form of thefuel rod is shown therein. The fuel rod 18 of FIGURE 2 is formed Ibypermanently elongating or stretching fthe fuel rod l18.01": FIG. l in ladirection parallel to its longitudinal axis. The stretching operationcan be effected by suitable gripping means associated with `thepresently .to :be described 'stretching apparatus and is carried outwith the fuelelement 18 in its horizontal position so that separatingmovement of the pellets 22 22 to the outer surface of the cladding tube20 where the j relative to -one lanother will occur before the pelletsare actually gripped by the walls of `the cladding tube 20. In `thismanner a maximum separation or space 45 is vformed between adjacentpellets. However, in those cases wherein a lesser separation 46 isdesired, the fuel rod 18 can be stretched in Ia vertical position withthe ,i

result that the pellets 22 will remain in contact under the influence ofgravity until the pellets are actually gripped `by the wall of thecladding tube 20. An exemplary apparatus for stretching the -fuelnod 18as foresaid, particularly in the horizontal position thereof, isdisclosed in a copending `application of R. K. McGeary and E. Frisch,entitled Compartmented Reactor Fuel Element, Serial No. 723,421, filedMarch 17, 1958, Vnow U.S. Patent No. 3,089,83'0, |and assigned to thepresent assignee. v

After the stretching operation, the clearance 42, which is providedinitially between the pellets 22 and the cladding tube 20, as shown inFIG. l, has disappeared, and those ywall portions intermediate the ends26 and 28 of the cladding tube 20 (FIG. 2) iare reduced in diameter sothat the inner surfaces of the aforesaid intermediate cladding tubeportion is in contact with the cylindrical surfaces of the pellets 22.This arrangement, of course, increases 'greatly the rate of heattransfer from the pellets heat is removed by a reactor coolant. Y

Moreover, the aforesaid stretching operation is continued until thelength ofthe cladding tube 20 is in-v ment of the invention, necked-downportions 48 and 50 adjacent theends 26 and 28 of the cladding tube 20"',whereby the pellets 22a and 22h (FIG. 2) are gripped by the claddingtube 20 and moved during the stretching operation a short distance 52`from the end plugs 34 and 30, respectively. v i

As the cladding tube 20 is stretched further, a localized necking orreduction 54V in cladding tube diameter occurs at the separations 46between adjacent fu-el pellets 22. The localized reductions 54 and theengagement of the pellets '22 with the inner surface. of lthe claddingtube 20 operates to maintain the pellets 22 in spaced relation duringcyclic operation of the fuel rod 18; that is to say,

at all times during heating and cooling thereof. Inasmuch as thecladding tube 20 is in physical contact with the pellets 22, thetemperature of the cladding tubes, during reactor operation isapproximatelyY equivalent to that of the adjacent cylindrical surfacesof the pellets.

Therefore, lthe cladding tube 20' will expand inthe longitudinaldirection Vslightly more than. the aforesaid cylin'- drical surfaces,due to its higher coefficient .of expansion,

assuming the claddingltubeand the fuel pellets fare fabricated fromstainless steel and uraniumoxide (U02), respectively. Therefore, thecircular edges of the pellets will not be moved into gouging contactwith the adjacent cladding tube reductions 54 by expansion of thepellets 22.

However, the pellets 22 will expand slightly more in the radialdirection than will the cladding tube 20 during Y reactor operation,since 'the central portions of the pellets are considerably hotter thanthe cylindrical surfaces thereof. `Thus the cladding tube 20'valwayswill remain in gripping contact with each of the pellets 22.Additionally, the aforesaid gripping contact will tend to prevent radialcracking of the fuel pellets to a much greater extent than if thepellets were free to expand 1 into a diametric clearance space.

In this arrangement of the invention, it is contemplated that the sumtotal of the separations 46 and 52 will be equivalent to or slightlygreater than the anticipated longitudinal diiferential expansion betweenthe fuel pellet column and the cladding tube 20 during operation of theneutronic reactor in which the fuel rods l' Vare utilized. Although thecylindrical surface of each pellet 22, will expand slightly less thanthe adjacent inner wall portion of the cladding tube Ztl the centralportions of the pellets 22 will expand longitudinally to a greaterextent during reactor operation, for these central portions, of course,`are then at Va considerably higher temperature. By providing a space`46 between each pellet 22, no one point of the cladding tube 29 can besubjected to the combined expansion of la plurality of pellets 22.However, if the cladding tube Zit' were not stretched to space lthepellets 22, the accumulation of chips, particles, and `other debris inthe clearance 42 (FIG. 1) could prevent movement of 1these one or morepellets relative to the cladding tube 2li so that the wall of thecladding tube ad jacent to these last-mentioned pellets would besubjected to the combined diiferential longitudinal expansion `of all ofthe pellets 22 below the pellet which is bound or gripped by theaforesaid accumulation of debris. over, bulging would occur at theaforesaid accumulation of debris inasmuch as the pellets 22 expanddifferentially in the radial direction relative to the cladding tube. Inthe stretched lfuel rod of FlG. 2, however, there are no clearancesbetween the pellets 22 and the cladding tube 20 to permit accumulationof debris, iand any such accumulation would be contained within `thespaces 46 and 52. inasmuch as any such debris is limited to thatresulting from only one or Itwo fuelV pellets and as the spaces orseparations 46 and 52 are not in communication, a `gross accumulation`of debris cannot occur in the fuel element of FIG. 2.

Being free to expand both upwardly and downwardly, within the claddingtube 20', the pellets 22 will not therefore impart any longitudinalstresses to lthe cladding tube 2l) las the fuel element 18 is heated. Onthe other hand, since the fuel pellets of known fuel rods are free toexpand only in the upward direction relative to the cladding tube, itwill be seen then that any localized `gripping accidentally occurning:during reactor operation would be directed toward the upward end ofithe fuel rod. Therefore, the degree of deformation which could beimparted to known types of fuel elements in the event that the.aforesaid binding between the cladding tube and fuel pellets thereofshould occur would be amplified or increased in accordance with the`distance of the localized binding from the bottom of these fuelelements.

More- From the foregoing it will be apparent lthat a novel and efficientform of fuel element adapted for use in a neutronic reactor has beendisclosed herein. Although the utility of vthe fuel rod has beendescribed chiefly in connection lwith a pressurized water neutronicreactor, it will be 'appreciated that the fuel rod can be adapted to anytype of heterogeneous reactor employing tubular fuel elements wherein aplurality of fuel pellets are inserted into a relatively closely fittingcladding tube.

Therefore, numerous modifications of the invention will occur to thoseskilled in this art without departing from the spirit and scope of theinvention. For example, it is obvious that the cladding tube and thefuel pellets need not be circular in cross-section, but can take someother configuration, for example la plate type fuel element, as long asthe cladding tube can be stretched or elongate-d to cause its inner wallsurfaces to grip the fuel pellets independently.

Accordingly, what is claimed as new is:

A method for producing a fuel element for a neutronic reactor, saidmethod comprising the steps of inserting a plurality of U02 fuel pelletswithin an elongated stainless steel cladding tube7 attaching an end plugto each end of said tube, the length of said tube and the manner ofattaching said end plugs being such that no Void space appears betweeneither of said end plulgs and its adjacent fuel pellet nor betweenadjacent fuel pellets, longitudinally stretching said cladding tube bypulling in opposite directions on each end plug to sepanate each of saidend plugs from its adjacent fuel pellet and each of said :fuel pelletsfrom adjacent fuel pellets, further stretching the wall of Vsaidcladding tube fto cause necking of the cladding tube so that thelocalized necking portion of the wall of the cladding tube engages eachof said fuel pellets independently yto maintain said separation, a-ndcontinuing said stretching until said separation is equivalent to theamount of differential thermal expansion between said fuel pellets andsaid cladding tube at the temperature of operation of said fuel element.-l

References Cited in the iile of this patent UNITED STATES PATENTS2,051,948 Inscho Aug. 25, 1936 2,110,752 wright Mar. s, 193s 2,441,858Watter May 18, 1948 2,798,848 Kingdom July 9, 1957 2,838,452 West etallune l0, 1958 2,864,758 Shackelford Dec. 16, 1958 2,874,459 HaldemanFeb. 24, 1959

